The invention relates to a method for machining material using laser radiation, in which the unfocused laser radiation, which is typically collimated, is focused through a focusing optics to a smaller beam cross-section, the optical axis of the focused laser radiation, referred to as beam axis, is directed at the material surface, whereby the beam axis is stationary relative to the material or is being moved along a processing path, which results from the focusing operation of the focused laser radiation and is held in the region of the interface of laser radiation and material, the laser radiation is partially absorbed on the interface such that due to induced material removal or induced material displacement the interface and thus also the laser radiation penetrate into the material, wherein the spacing of the beam waist from the upper or lower side of the interface in the axial direction corresponds to at most triple the value of the penetration depth of the interface into the material. The laser radiation used for machining the material can consist of one or more beams that are generated using one or more radiation sources.
The invention furthermore relates to a respective device for machining material using laser radiation. Methods for machining material using laser radiation, where the focused laser radiation penetrates the material are, for example, drilling, removing, cutting and welding. While the penetration of the interface and thus also of the laser radiation into the material in the first three methods mentioned occurs by material removal in the form of melting, evaporating, sublimating or disintegrating, in the case of laser welding a vapor capillary is generated in a molten bath of the material that displaces the molten bath and through which the laser can penetrate into the material. In drilling, the material may remain stationary relative to the laser beam; the other methods utilize a relative movement of the laser radiation relative to the material. All methods mentioned can be aided by using a process gas flow that can consist of reactive but also of inert gases and said gas flow is used, for example, for expelling the molten or evaporated material components or for influencing the surface properties of the interface and of adjacent material regions.
All named methods have in common that the interface typically has a grid ratio above 1, i.e., the beam diameter and the width of the interface are smaller than the penetration depth into the material. Thus, significant importance is not only placed on the radiation properties on an, at best, minimally deformed material surface, as is the case with surface-treating laser methods, but rather on the radiation properties of the entire propagating interface, which penetrates the material, across the entire propagation distance of the radiation between the upper side of the interface and the lower side of the interface.
Until now—in addition to the radiation power—the beam diameter in the beam waist, often referred to as the focal diameter, and the Rayleigh length of the radiation in the waist region, defined as the distance along the beam axis measured from the beam waist to the point, where the beam cross-sectional area has doubled, have been considered significant radiation properties. In addition, the influence of the power density distribution in the beam cross-section (also referred to as intensity distribution) in particular in the beam waist on the machining result is considered important, although the exact effects of the intensity distribution are still not known sufficiently. Until now, the distribution of the directions of propagation of various radiation components in the focused laser radiation and their influence on the efficiency of the machining process and on the machining result have not been taken into account at all.
It is known that high-performance CO2 lasers (10μ emitters) with a wavelength of approx. 10 μm and with a laser power of 1-15 kW are used industrially for laser material machining applications (for example, macro applications for sheet metal in a range from 1 mm to 30 mm). In addition, rod lasers, fiber lasers and disc lasers (1μ emitters) with wavelengths of approx. 1 μm and with a laser power of 1-8 kW are used. In particular, these radiation sources offer economic advantages and are, therefore, being increasingly used. However, it has become apparent that in particular when cutting with laser radiation, the achievable machining quality is dependent, for example, on the radiation source used (fiber lasers, disc lasers (1μ emitters), gas lasers 10μ emitters) and, for example, on the thickness of the sheet to be cut and the travel speed.
Current developments in material machining using lasers aim at a further increase in the machining speed, an increase in the achievable machining depths and/or of the machined material thicknesses, an improvement of the process robustness, an avoidance of process instability and not least an increase in the achievable machining quality. For this reason, increasingly greater laser powers and systems with a high-quality drive technology are introduced in manufacturing. The development aims at expanding the technical restrictions of process control.
Quality of the Machining Geometry Using the Example of Cutting:
In addition to low roughness and burr-free bottom sides as well as products free of oxides, evenness and right angles are significant quality requirements for the cut edge. The following points should, therefore, be considered:
With an increasing sheet metal thickness, the cut edge exhibits increasingly rough gouges, which appear in particular in the lower part of the cut edge (e.g., abrasion kerf) and occur increasingly when the cutting gas pressure is too small, the kerf is too narrow or the speed is too high.
In particular with low or high feed rates, the melt is not fully removed from the bottom edge. The attached and then solidifying melt forms the undesired burr. The mechanisms by which such burrs are generated are understood only in part. They are, among other things, connected to the formation of gouges.
From experiential observations, it became known that the undesired and today unavoidable gouges can change at a region of a certain removal depth (or cutting depth) from small peak-to-valley roughness depths to significantly greater values. This change can occur in a region of the cutting depth that is small compared to the thickness of the workpiece. During a cut, this region may occur at varying depths on the cut edge (or removal edge, respectively).
Absorption of the Laser Radiation: